1. Field of the Invention
This invention relates to a multilayer wiring substrate which is manufactured through a firing process and holds therein passive circuit elements such as a resistive element and an inductor element, and to a method of manufacturing the same.
2. Description of the Related Art
When a thick-film resistive element is formed within multilayer wiring substrate for a hybrid integrated circuit, conventionally, as shown in FIG. 12, a plurality of conductive patterns 2 including a pair of resistive element terminal electrodes 2a are formed on an insulating substrate 1 by printing and firing steps using conductive paste. Then, a thick-film resistive element 3 is formed across the pair of the terminal electrodes 2a by printing and firing steps using resistive paste. The thick-film resistive element 3 and the conductive patterns 2 are covered with a protective overcoat glass (not shown).
However, in the construction described above, the terminal electrodes 2a and the thick-film resistive element 3 are arranged in a two-dimensional state, so that a region indicated with slant lines in FIG. 12 becomes a dead space for the thick-film resistive element 3. As a result, the area necessary for arranging the thick-film resistive element 3 becomes large, resulting in increase in entire area of the substrate. To solve this problem, recently, a substrate technique for forming a multilayer wiring substrate has been adopted. An example of this kind of substrate technique will be explained referring to FIGS. 14A to 14D.
First, as shown in FIG. 14A, a plurality of conductive patterns 5 are formed on an insulating substrate 4 by printing and firing steps using conductive paste. The conductive patterns 5 includes a pair of terminal electrodes 5a for a resistive element. The insulating substrate 4 is made of inorganic material. Next, as shown in FIG. 14B, a thick-film resistive element 6 is formed to be connected to the terminal electrodes 5a on the insulating substrate 4 by printing and firing steps using resistive paste.
After that, as shown in FIG. 14C, an insulating layer 7 made of for example glass material is formed on the insulating substrate 4 to have via holes 7a for exposing the terminal electrodes 5a and parts of the conductive patterns 5. As shown in FIG. 14D, then, terminal electrodes 8 filling the via holes 7a and conductive patterns 9 disposed on the insulating layer 7 to be connected to the terminal electrodes 8 are formed by printing and firing steps using the conductive paste. Accordingly, a thick-film multilayer wiring substrate for a hybrid integrated circuit is completed. Incidentally, FIGS. 14A to 14D show the process for forming the thick-film wiring substrate having a two-layer structure in a stepwise manner; however when a thick-film multilayer wiring substrate having more than three layers is manufactured, after the printing and firing steps are carried out to form the terminal electrodes 8 and the conductive patterns 9, the steps shown in FIGS. 14A to 14D are repeatedly carried out.
In the process described above, in the firing step for the insulating layer 7, a temperature is raised to approximately 850.degree. C.-900.degree. C. As opposed to this, a normal temperature range of the thick-film multilayer wiring substrate is comparatively low (for example -40.degree. C.-150.degree. C.). Accordingly, residual stress is produced due to a difference in thermal expansion coefficient between the thick-film resistive element 6 and the insulating layer 7.
In the conventional structure, as shown in FIG. 13, the thick-film resistive element 6 swells up at overlapping portions with the terminal electrodes 5a. Therefore, the residual stress is liable to concentrate on swelling portions A of the thick-film resistive element 6 to cause cracks. Likewise, the insulating layer 7 has portions B corresponding to the swelling portions A. The portions B of the insulating layer 7 have a thickness thinner than that of the peripheral portion thereof and slightly swell up along the swelling portions A of the thick-film resistive element 6. Accordingly, cracks may be generated at the portions B and may grow toward the thick-film resistive element 6. When the thick-film resistive element 6 has the cracks therein, its value of resistance deviates from the target value thereof, resulting in decrease in reliability. This kind of problem occurs in the so-called green sheet lamination method as well.